1. Field of the Invention
Exemplary embodiments of the present invention relates to an image forming apparatus that calculates an index indicative of toner chargeability of carrier in developer and supplies an appropriate amount of carrier to a developing unit based on the calculated index.
2. Discussion of the Related Art
Known image forming apparatus developing units generally develop a latent image with developer consisting essentially of toner particles and carrier particles, in which only the toner particles are consumed during image development. As the concentration of toner in the developer decreases due to toner consumption during image development, fresh toner particles are added to the developer.
In such known developing units, a toner charge amount within the developing unit can be destabilized by toner deterioration caused by mechanical stress repeatedly applied to the developer by agitating the carrier particles inside the developing unit. At this time, a toner charge amount can either decrease or increase, generally depending on what type of images are most often output.
Specifically, with frequent output of images having a high image area ratio, that is, images in which the recorded image area occupies a large proportion of the total surface area of the recording medium, carrier particles can easily be degraded due to a phenomenon that is referred to as “toner spent”, in which the toner component gets fixed onto the surface of the carrier particles. The spent carrier particles can reduce toner chargeability that can be developed by frictional contact with the toner particles, and therefore decrease a toner charge amount in the developing unit. If the toner charge amount is too small, the electrostatic adhesion force exerted between the toner and the carrier weakens, resulting in output of defective images due to background contamination caused by adhesion of toner to a non-imaging area of the recording medium. In addition, the interior of an image forming apparatus incorporating the developing unit can be contaminated due to toner scattering.
By contrast, with frequent output of images with a low image area ratio, the carrier particles in the developer can suffer peeled coating, in which a coating film on the surface of the carrier particle is peeled off. Carrier particles with peeled coating encounter or create increased resistance when in frictional contact with the toner, thereby increasing a toner charge amount in the developing unit. If the toner charge amount is too large, an excessive electrostatic adhesion force is exerted between the toner particles and the carrier particles can dilute image density of the formed image, resulting in faint or washed-out images.
In a known image forming apparatus, after a particular development γ based on the image density of a test patch image and the toner density is measured at a given time it is compared with a known initial developer and new carrier or new developer is gradually supplied to a developing unit based on the measurement results. The development γ is defined as a slope of a line representing a relation between a development potential and an amount of toner adhesion per unit area with respect to a toner image, as shown in FIG. 1. The term “development potential” means a difference in potential between the potential of a latent image carried by a latent image carrying member such as a photoconductor and a developing bias that is applied to a developer bearing member such as a developing roller.
The development γ may vary according to the toner concentration in developer. However, if conditions other than the toner concentration are held constant, a relation that is indicated by a predetermined function arises between the degree of the development γ and the toner concentration. Therefore, when the development γ that corresponds to a particular toner concentration is specified and determined based on the function, any deviation from the expected result between the development γ and the toner concentration can be regarded as a change in the toner chargeability due to deterioration of the carrier particles. That is, the development γ may serve as an index that is a value indicating the toner chargeability of a carrier particle.
The above-described known image forming apparatus stores a function confirmed by previous tests in a data storage unit. When the development γ is measured, a stored value for the development γ as given by the stored function for the measured toner concentration is specified. If there is any difference between the specified value and the measured value of the development γ, a certain amount of new carrier or new developer that corresponds to the difference between development γ and the toner concentration is supplied to the developing unit.
The above-described operation of supplying new carrier or developer may result in an overflow of used carrier or developer, that is, an action that a certain amount of used developer that corresponds to the amount of new carrier or developer supplied to the developing unit overflows from the developing unit. The supply and overflow of carrier or developer can change new and used carriers according to degree of carrier deterioration, thereby stabilizing the toner charge amount.
In the measurement of the development γ, respective given toner images are developed under conditions of different development potentials, and amounts of toner adhesion with respect to the respective toner images are detected by a photosensor. Based on the detection results, a slope of a line indicating a relation between the development potential and the amount of toner adhesion is obtained as a development γ.
However, the development γ may vary significantly in accordance with humidity fluctuation as well as carrier deterioration. Therefore, in the above-described known image forming apparatus, when the development γ varies according to the humidity fluctuation, the change can be misinterpreted as carrier deterioration, and such a false detection can make it difficult to ascertain the toner chargeability of carrier accurately.
Not only the development γ that can serve as an index of the toner chargeability but also the toner chargeability itself can vary according to humidity. Therefore, not only when the development γ but also when any parameter is used as an index of the toner chargeability, a problem similar to the above-described problem can occur.